1. Field of the Invention
The present invention relates to the manufacture of a multi-grade lubricating grease having particularly desirable high temperature properties. More specifically, this invention is directed to a method of manufacturing an improved multi-grade grease comprising a lubricating base oil, a fatty amine, a bispyrrolidone derivative and a compound of lithium or other metal.
2. Discussion of Related Art
Greases are often used in place of oils for lubrication where the lubricant is required to maintain its position in a mechanism and where opportunities for frequent relubrication may be limited or not economically justified.
Over the years, a variety of thickening agents for greases have been developed including the alkali salts of fatty acids, clays, polyureas, asbestos, carbon black, silica gels, aluminum complexes, polymers, phthalocyanine, indanthrene, etc. Despite the number and variety of such thickeners, alkali metal soaps are used as the thickening agent in over 90 percent of worldwide grease production. The only non-soap thickeners which have achieved commercial importance are the aluminum complexes, clays and polyureas, and then only to a very limited extent.
The soap thickeners most often used are derived from the saponification of fats and oils by lithium and calcium hydroxides, although the sodium and barium soaps have been used in smaller amounts for special applications. The fats and oils are mostly mixtures of C.sub.16 and C.sub.18 fatty acid precursors with the preferred soap being lithium 12-hydroxystearate. This preferred lithium soap thickener is present in over 50 percent of all greases and most all premium multi-purpose greases. Lithium soap greases are described and exemplified in many patents, including, for example, U.S. Pats. Nos. 3,758,407; 3,791,973; and 3,929,651.
Greases generally must exhibit good anti-rust and anti-wear properties. In addition, greases should not exhibit oil-soap separation or excessive softening in use. Relatively firm greases (e.g. National Lubricating Grease Institute #2 grade greases) have proven useful in applications requiring good stay-in-place characteristics, while relatively soft greases (e.g., NLGI#0 grade greases) have proven particularly useful where good adhesion of the grease to metal surfaces is desired. For wheel bearing applications, although a general application grease such as NLGI#2 grade is suitable, softer NLGI#0 grade greases are unsuitable as they do not have good stay-in-place properties and, therefore, tend to bleed from the bearings. Heretofore thickeners have not been formulated which give multi-grade characteristics--namely the desirable stay-in-place properties of a firm grease and the metal adhesion properties and fluidity of a soft grease. Such a multi-grade grease would be particularly useful in automotive wheel bearings, where excessive softening could lead to braking failures and good adhesion of the grease to the metal surfaces is desired to provide the required lubrication. However, to date, there are no commercially available greases which have the combination of desirable stay-in-place properties of a firm grease and the metal adhesion properties and fluidity of a soft grease.
Recently, a new type of chemistry for preparing a grease thickener has been disclosed in U.S. Pat. No. 4,253,979 to Alexander et al., the disclosure of which is incorporated herein by reference. More specifically, patentees disclose therein a lubricating grease composition comprising a base oil, conventional additives and a thickener which in turn comprises the reaction product of a pyrrolidone with a fatty amine to form a fatty monoamide. The fatty monoamide subsequently is reacted with an hydroxide, such as lithium hydroxide, to make the thickener salt. However, the pyrrolidone compound is reacted with the fatty amine under conditions conducive to promoting the complete reaction of the reactants (see the reference to "appropriate molar proportions" at column 3, lines 38-40 and the examples). In addition, all of the examples in Alexander et al. show that the NLGI #2 grade greases prepared pass the high temperature (163.degree. C.) wheel bearing tests (the penetration values in each example correspond to NLGI #2 grade greases). However, greases prepared according to the teachings of the U.S. Pat. No. 4,253,979 would not pass the 163.degree. C. wheel bearing test when "oiled back" (i.e., additional base oil is added) to the softer NLGI #0 grade greases. This result is not unexpected as all commercially available greases demonstrate this behavior.
Therefore, it would be desirable to have available a method of producing a NLGI #0 grade grease that has the high temperature stay-in-place properties of a NLGI #2 grade grease such that the #0 grade grease will consistently pass the 163.degree. C. wheel bearing tests (ASTM test D-1243 at 163.degree. C.).
SUMMARY OF THE INVENTION
According to the present invention, it has now been discovered that the procedures described in U.S. Pat. No. 4,253,979 for forming a lubricating grease composition can be modified to form a different grease having different properties than that taught by patentees. More specifically, a NLGI #0 grade grease having the high temperature stay-in-place properties of an NLGI #2 grade grease can now be produced by reacting a bispyrrolidone with a fatty amine, followed by the addition of an hydroxide (preferably lithium hydroxide) before the reaction of the bispyrrolidone with the amine approaches completion; i.e., when the unreacted fatty amine content ranges between about 33 and about 66 wt. %, preferably between about 36 and 66 wt. % and more preferably between about 40 and about 55 wt. % of the fatty amine originally added.